Molecular sieves have found a wide range of applications within refinery processes and other processes for manipulating petroleum streams. Some applications are catalytic in nature, while other applications focus on the ability of molecular sieves to selectively adsorb certain molecules within a gas stream.
One example of selective adsorption of molecules from a gas phase stream involves the use of a zeolite or another microporous material to remove contaminants from a stream containing hydrocarbons or other small gas phase organic molecules. For example, many natural gas streams contain at least some CO2 in addition to the desired CH4. Additionally, many refinery processes generate a gas phase output that includes a variety of species, such as CH4 and CO2, that are gases at standard temperature and pressure. Performing a separation on a gas phase stream containing CH4 can allow for removal of an impurity and/or diluent such as CO2 or N2 under controlled conditions.
For example, U.S. Patent Application Publication No. 2008/0282885 describes systems and methods for removing CO2, N2, or H2S from a gas mixture containing CH4, such as natural gas, using a swing adsorption process. One type of adsorbent that can be used in the swing adsorption process is an 8-ring molecular sieve, such as a DDR-type zeolite.
One problem hindering the commercial application of 8-ring molecular sieves, such as DDR-type zeolites, in the sorptive separation of impurities, such as CO2, from gas mixtures containing CH4 is that the diffusivity of the methane into most 8-ring molecular sieves is too high, often by an order of magnitude. There is therefore a need to reduce the methane diffusivity of these molecular sieves while at the same time maintaining the capacity of the molecular sieve for CO2 uptake.